The linear propagation of light in a medium is typically dependent upon the wavelength of the photons being propagated through the medium since the various interactions between the medium (typically the atoms that constitute the medium) and the photons are wavelength dependent. An example of such wavelength dependence is where the index of refraction of a medium is a function of the wavelength such that the phase accumulated via propagation is wavelength dependent. Such chromatic dispersion results in different optical frequencies (or wavelengths) in a pulse traveling at different speeds. Therefore, the shape of the pulse propagating in a dispersive medium changes.
Chromatic dispersion can be an important source of impairment in an optical telecommunication system, and must therefore be properly characterized in order to be compensated for. For example, chromatic dispersion can cause successive pulses in a communication channel to overlap, degrading the quality of the recovery of the information carried by the pulses.
A variety of approaches for characterizing chromatic dispersion are known in the art. Test-plus-reference spectral interferometry is one approach which directly measures the phase difference between a pulse transmitted along a test path (i.e. through a device under test) and a reference path. Such an approach may be accurate for short devices but can not be applied to long devices, such as the fiber spans that constitute optical networks. Also, the requirement of a reference path without dispersion precludes the characterization of installed fiber spans.
Other approaches use RF phase-shift techniques to determine the phase-shift induced by propagation in a device using an intensity-modulated narrow-band source. Such techniques are typically accurate, however, they cannot be used to characterize a large range of dispersions with the same setup, and they sometimes require a reference path.
Still other approaches employ optical time-of-flight techniques to obtain the group delay from the propagation time of short pulses at various optical frequencies. These approaches suffer from accuracy and resolution problems, since the frequency and time resolutions are inversely proportional. Practically, it is desirable to have the ability to measure both large and small dispersions, to perform self-referencing measurement without a reference path and to characterize devices with high insertion losses.